Through-substrate optical imaging devices include a radiation source outputting radiation that will be transmissive through the work object. Furthermore, the known devices include an imaging system configured for capturing inspection information from the radiation source through the work object.
There is a need for improved imaging systems, particularly inspection systems in the manufacturing of semiconductor devices to determine whether or not certain structures are present. Typically, the structures are defects. While the inventive subject matter may be used in various imaging or inspection applications, it will be illustrated herein in the context of a Micro Electro Mechanical System (MEMS) inspection.
The general structure of a Micro Electro Mechanical System (MEMS) device involves a device wafer, electronic or mechanical components located on the device wafer, a barrier surrounding the edge of the device wafer, and a cap wafer. During the assembly of the MEMS device, a frit is affixed to the bottom of the cap wafer. The cap wafer is then pressed down onto the device wafer causing the frit to create a bond with the device wafer.
In one method of determining if the device has seal integrity, a beam of infra-red (IR) or near infra-red (NIR) light is projected either through the top (toplight or Si Reflect Method) or the bottom (backlight, transmissive or Si-Thru method) of the wafer. On the top side of the wafer is a camera or photodetection device.
The standard method to detect seal defects is to project infrared or near infrared collimated light orthogonally to the device surface on one side to check for seal integrity. When the wafer is illuminated with infrared or near infrared collimated light, the seal extents and gross voids in the seal are easily detected.
Unfortunately, in this method illumination of the device wafer does not highlight subtle delamination between the seal and the device wafer to which the seal is bonded. This failure to highlight the subtle defects results because delaminations do not significantly affect transmission of the light passing through the device wafer and the frit. Accordingly, there is a need for improved inspection systems in MEMS inspections and other inspection applications.